Current overhead projection media typically consists of a transparent film with printing of some color and density. The projection film is placed on a light source where light is transmitted through the projection film and collected by the collection lens. The collection lens then projects the light to a projection screen. The projected image is much larger than the original projection film, making this system suitable for multiple viewers, as in a conference room or a classroom.
Unfortunately, the current projection film has some drawbacks. Typically, the projected image will be black or colored features set against a brightly illuminated white background of transmitted light, for example, black text (essentially no light transmission) on a background of maximum light projection.
Because the bright white background of the projected image is bright, the viewers can not easily view light colors printed on the transparency. In addition to the light coming from the overhead projector and comprising the image information, ambient light will be incident on the projection screen and reflected by this screen towards the audience. To obtain an image on the projection screen that is sufficiently rich in contrast during a presentation, at which illustrations, graphs and the like are shown by means of an overhead projector, the audience space will have to be darkened considerably. This darkening, which is often necessary more than once during a lecture, is a disturbing and detracting operation. Moreover, the darkening reduces the quantity of light for the audience. This may be troublesome if they want to make notes, for example. Moreover, the audience is less well visible to the speaker, due to the darker ambience. Until now, the darker ambience has been accepted as a necessity and no useful overhead presentation system has been conceived which can work with a satisfactory contrast without darkening. An increase of the power of the lamp in the illumination system, so that the signal light will be considerably more intensive than the ambient light, is not a good solution because the lamp must then be cooled thoroughly, which necessitates the use of a fan causing a disturbing noise, which is troublesome, particularly during lectures.
It would be advantageous to have a transparency film that could be projected with bright and dark areas with colors.
Light diffusing elements that scatter or diffuse light generally function in one of two ways: (a) as a surface light diffusing element utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk light diffusing element flat outer surfaces and embedded light-scattering elements. The surface light diffusing elements normally utilize the rough surface, typically with a lens, exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. The bulk diffuser diffuses the light within the film. Examples are small particles, spheres, or air voids of a particular refractive index are embedded another material with a differing refractive index.
Diffusion is achieved by light scattering as it passes though materials with varying indexes of refraction. This scattering produces a diffusing medium for light energy. There is an inverse relationship between transmittance of light and diffusion and the optimum combination of these two parameters is desired for each application.
U.S. Pat. No. 5,852,514 (Toshima et al.) describes a light diffusing element comprising a light diffusion layer including acrylic resin and spherical particles of polymethyl methacrylate on a transparent support. Whereas this film would diffuse the light efficiently, the polymers used have high glass transmission temperatures and would therefore be difficult to melt the spherical particles completely to create areas of specular transmission. When projected these not completely melted lenses would diffuse a portion of the light lowering the brightness of the printed, more specular, projected areas and thus lowering contrast of the overhead projected image.
In U.S. Pat. No. 6,177,153, oriented polymer film containing pores for expanding the viewing angle of light in a liquid crystal device is disclosed. The pores in U.S. Pat. No. 6,177,153 are created by stress fracturing solvent cast polymers during a secondary orientation. In example 1 of this patent, the reported 90% transmission includes wavelengths between 400 and 1500 nm integrating the visible and invisible wavelengths, but the transmission at 500 nm is less that 30% of the incident light. Such transmission values would cause unacceptably dark images when projected onto a projection screen.
U.S. Pat. No. 3,763,779 (Plovan) discloses a method for copying an image by selectively coalescencing microporous voids in a voided film to create areas of transparency. The method has limitations in that to produce a copy in the voided film, an original must be used and the original must be of a particular material and format. It would be desirable to have a process to selectively coalesce voids using an electronic file as the template. The film has voids throughout the thickness of the film so that to make an area of the film transparent, the voids throughout the thickness of the film must all be coalesced or melted. This requires a substantial amount of energy making this method expensive, time consuming, and difficult.
U.S. Pat. No. 6,386,699 (Ylitalo et al.) discloses an embossed media for use as an inkjet receiver. This receiver could be part of a transparency media for overhead projection. The embossed surface is used to catch the inkjet materials and allow for drying time. The inkjet process does not change the structure of the embossed surface of the receiver media and would therefore the projected image as all the same light intensity on the screen when lit by the overhead projection system.
U.S. Pat. No. 4,497,860 (Brady) discloses a method of using a linear prism array to project areas onto an overhead projection screen of high and low illumination. The method involves using a sheet with a linear prism on one side and heat to create non-refracting areas that show up as bright areas when projected. The light that passes through the still intact linear prism array is deflected away from the collection lens and is viewed as darker areas of illumination on the screen. By adding a diffraction grating and another linear prism array, the method can create colored images where the projected image can have at most two colors (one non-refracting area color and one refracting area color), for example, purple text on a green background. It would be desirable if more than two colors could be displayed by the overhead projection system at once. An undesirable aspect is the process of molding of the linear prism array must be very exact leading the method to become cost prohibitive. A small difference in molding temperature or time yields vastly different qualities of projection media. In addition, when the linear prisms are melted, if the melting tool is not correctly aligned with the linear prism array, then some of the prisms will be melted partially distorting the projected image in color and density because the half-melted lens array will not deflect light correctly. A moiré pattern may be visible when the films are projected because of the use of a repeating surface pattern, especially when the one linear prism array is placed in conjunction with another linear prism array or diffraction grating. This moiré pattern is undesirable as it detracts from the projected material.
U.S. Pat. No. 5,369,419 (Stephenson et al.) describes a thermal printing system where the amount of gloss on a media can be altered. The method uses heat to change the surface properties of gelatin, which has many disadvantages. Gelatin cannot achieve high roughness averages, thereby having a low distinction between the matte and glossy areas of the media. This small distinction between the matte and glossy states leads to a low signal to noise ratio and when projected, creates lower contrast ratios. Gelatin also is very delicate, scratch prone, and self-healing, and so it tends to flow over time thus changing its surface roughness and other properties with time especially in high humidity and heat, and is dissolved if placed in water. Also, gelatin has a native yellow color, is expensive, and is tacky, sticking to other sheets and itself. It would be desirable to use a material that had no coloration, is more stable in environmental conditions, and could have a higher surface roughness.
U.S. Pat. No. 2,739,909 relates to a heat-sensitive recording paper created by overcoating black-colored paper with a continuous thermoplastic resin material containing microscopic voids dispersed throughout the resin. The coating layer is opaque, but becomes transparent by the localized action of a stylus using either heat or pressure or both to disclose the black color of the support. There is a problem with this element in that the manner of obtaining the voids is complicated which involves carefully controlled drying conditions of emulsions. Another disadvantage with this design is that while the coating turns transparent, the coating is formed on a black colored paper that makes the invention useless for a projection application.